A good range for the capacitor is 200 - 500 nF, depending on the work coil inductance you are targeting. Higher values lead to less inductance to keep the working frequency between 750 and 1150 kHz. I made one with a single turn coil and it was struggling to couple significant power into smaller objects, so I would aim for a two turn coil. I found
https://hamwaves.com/inductance/en/index.html to give pretty accurate estimations of inductance for a given coil geometry.
Also note that the wiring between the cap and the coil adds to the work coil inductance, and since we are aiming at low 100s of nH here this can be significant. The original design uses very close spacing and wide parallel conductor plates to minimize this inductance, allowing most of the heating power to go into the coil and not the wiring between the coil and cap.
As an example, if we want to target 800 kHz with 330 nF, we get L = 1/((2*pi*800e3)^2*330e-9)) = 120 nH. If we leave 20 nH for the wiring then we need a 100 nH coil. The hamwaves calculator indicates that a 25 mm diameter, two turn coil with a length of 10 mm will get us close.
